@GNH 

If you have power down in the bunk house you could buy the amount of outdoor rated ethernet cable needed to reach the side of the bunk house from to the main Ax20,

buy another AX20 and run that in AP mode. 

Me personally that would be how I would do it just for the stability of having a more reliable connection via ethernet. 

I would like to think that should give you the desired coverage down to the Dock area. 

Obv there are outdoor range extenders (not sure if can be run in AP mode as never dealt with them personally but I beleive you would still need power (for POE) aswell as the ethernet run to the bunk house/dock. 

You could do a wireless bridge set up but still would need power in the bunk house to power the repater, 

obv again you would have to run ethernet depending where you want the nodes/repeater  to be either on the side of the bunk house or off a mount down at the Dock. 

There are a few options You can choose from that can suit your needs/budget and skills according to your DIY skills but will be a case of weighing up the pros and cons aswell as the costs of each setup. 

Not quite the same situation but simular where I have 3 Ax72 in my property, 

I have 1 main unit and 2 satellites running in AP mode. 

I have one down the bottom of my garden connected via outdoor rathed ethernet to my main unit in my outhouse that gives me access to my home WiFi covering the bottom of my garden plus further out. 

Just done a quick search and it would cost about £100-120 for another ax20 and the 100m of outdoor ethernet, 

You could save money depending if you choose a different cheaper router plus if you want to hard wire anything in the bunk house you will have some ethernet ports on the back of the router should you need/want them.